GENETICS OF HOST-CACTUS RESPONSE AND LIFE-HISTORY EVOLUTION AMONG ANCESTRAL AND DERIVED POPULATIONS OF CACTOPHILIC DROSOPHILA MOJAVENSIS

The extent of host-specific genetic variation for two life-history traits, egg to adult developmental time and viability, and one morphological trait closely tied to fitness, adult thorax size, was exposed by employing a nested half-sib/full-sib breeding design with Baja and mainland populations of Drosophila mojavensis recently extracted from nature. This study was motivated by the presence of substantial variation in life histories among populations of D. mojavensis that use the fermenting tissues of particular species of columnar cacti for feeding and breeding in the Sonoran Desert. Full-sib progeny from all sire-dam crosses were split into cultures of agria cactus, Stenocereus gummosus, and organ pipe cactus, S. thurberi, to examine patterns of genotype-by-environment interaction for these fitness components. Baja flies expressed shorter egg-to-adult developmental times, higher viabilities, and smaller body sizes than mainland flies consistent with previous studies. Significant sire and dam components of variance were exposed for developmental time and thorax size. Genotype-by-environment interactions were significant at the level of dams for developmental time and nearly significant for viability (P = 0.09). Narrow- and broad-sense heritabilities were influenced by host cactus, sex, and population. No strong pattern of genetic correlation emerged among fitness components suggesting that host-range expansion has not been accompanied by formation of coadapted life histories, yet the ability to estimate genetic correlations and their standard errors was compromised by the unbalanced nature of the data set. Genetic correlations in performance across cacti were slightly positive, evidence for ecological generalism among populations explaining the observed pattern of multiple host cactus use within the species range of D. mojavensis.     

GENETICS OF SEXUAL ISOLATION AND COURTSHIP DYSFUNCTION IN MALE HYBRIDS OF DROSOPHILA PSEUDOOBSCURA AND DROSOPHILA PERSIMILIS

Despite the importance of sexual isolation to speciation, few studies have analyzed the genetic basis of interspecific mating discrimination, particularly using hybrid males. In this study, I investigated the genetic basis of sexual isolation using male hybrids of Drosophila pseudoobscura and D. persimilis. Hybrid male mating success was caused by interactions between the X-chromosome and autosomes (or Y-chromosome), and different arms of the X-chromosome contributed to mating success with females of each species. Further, although there was an X-chromosome component to mating success, its magnitude was not disproportionately large when compared with the proportion of the genome contained on this chromosome. Some hybrid males courted with an anomalously low intensity, so I simultaneously mapped the genetic basis of this “courtship dysfunction.” The courtship dysfunction was caused by an interaction between the left arm of the X-chromosome in D. persimilis with the autosomes or Y-chromosome from D. pseudoobscura. Anomalous courtship behavior in interspecific hybrids can obscure the conclusions of studies of the genetics of sexual isolation, so courtship intensity should be evaluated in all such investigations.     

THE GENETIC BASIS OF SEXUAL ISOLATION BETWEEN DROSOPHILA MELANOGASTER AND D. SIMULANS

The genetic analysis of sexual isolation between the closely-related species Drosophila melanogaster and Drosophila simulans involved two experiments with no-choice tests. The efficiency of sexual isolation was measured by the frequency of courtship initiation and interspecific mating. We first surveyed the variation in sexual isolation between D. melanogaster strains and D. simulans strains of different geographic origin. Then, to investigate variation in sexual isolation within strains, we made F₁ diallel sets of reciprocal crosses within strains of D. melanogaster and D. simulans. The F₁ diallel progeny of one sex were paired with the opposite sex of the other species. The first experiment showed significant differences in the frequency of interspecific mating between geographic strains. There were more matings between D. simulans females and D. melanogaster males than between D. melanogaster females and D. simulans males. The second experiment uncovered that the male genotypes in the D. melanogaster diallel significantly differed in interspecific mating frequency, but not in courtship initiation frequency. The female genotypes in the D. simulans diallel were not significantly different in courtship initiation and interspecific mating frequency. Genetic analysis reveals that in D. melanogaster males sexual isolation was not affected by either maternal cytoplasmic effects, sex-linked effects, or epistatic interaction. The main genetic components were directional dominance and overdominance. The F₁ males achieved more matings with D. simulans females than the inbred males. The genetic architecture of sexual isolation in D. melanogaster males argues for a history of weak or no selection for lower interspecific mating propensity. The behavioral causes of variation in sexual isolation between the two species are discussed.     

SPECIES HYBRIDS IN THE LABORATORY BUT NOT IN NATURE: A REANALYSIS OF PREMATING ISOLATION BETWEEN DROSOPHILA ARIZONAE AND D. MOJAVENSIS

Understanding speciation relies critically on the identification of mechanisms responsible for maintaining species integrity (i.e., reproductive isolation) especially when closely related species are sympatric in nature. Studies of reproductive isolation in Drosophila often involve laboratory mating experiments that assume that patterns of mate choice in the laboratory are similar to those in the wild. Two sibling species, Drosophila arizonae and D. mojavensis , known to exhibit low levels of interspecific hybridization in the laboratory, but not in nature, were used in multiple-choice mating trials using various mating chamber designs as well as synthetic and natural media for developing larvae and courting adults. Sympatric populations were more sexually isolated than allopatric ones, consistent with past studies, and all experimental variables tested (chamber size, host plant presence and rearing substrates) had significant effects on levels of premating isolation between these species. Flies reared on cactus showed increased premating isolation versus those reared on synthetic laboratory food as did providing fermenting host plant tissue during mating trials. Also, surprisingly, smaller mating chambers led to an increase in premating isolation versus larger containers. The design of these types of mating trials is thus critical to understanding how mating behaviors in the laboratory are related to those in natural populations.     

WOLBACHIA AND THE EVOLUTION OF REPRODUCTIVE ISOLATION BETWEEN DROSOPHILA RECENS AND DROSOPHILA SUBQUINARIA

Endosymbiotic bacteria of the genus Wolbachia are widespread among insects and in many cases cause cytoplasmic incompatibility in crosses between infected males and uninfected females. Such findings have been used to argue that Wolbachia have played an important role in insect speciation. Theoretical models, however, indicate that Wolbachia alone are unlikely to lead to stable reproductive isolation between two formerly conspecific populations. Here we analyze the components of reproductive isolation between Drosophila recens, which is infected with Wolbachia, and its uninfected sister species Drosophila subquinaria. Laboratory pairings demonstrated that gene flow via matings between D. recens females and D. subquinaria males is hindered by behavioral isolation. Matings readily occurred in the reciprocal cross (D. quinaria females × D. recens males), but very few viable progeny were produced. The production of viable hybrids via this route was restored by antibiotic curing of D. recens of their Wolbachia symbionts, indicating that hybrid offspring production is greatly reduced by cytoplasmic incompatibility in the crosses involving infected D. recens males. Thus, behavioral isolation and Wolbachia-induced cytoplasmic incompatibility act as complementary asymmetrical isolating mechanisms between these two species. In accordance with Haldane's rule, hybrid females were fertile, whereas hybrid males invariably were sterile. Levels of mtDNA variation in D. recens are much lower than in either D. subquinaria or D. falleni, neither of which is infected with Wolbachia. The low haplotype diversity in D. recens is likely due to an mtDNA sweep associated with the spread of Wolbachia. Nevertheless, the existence of several mtDNA haplotypes in this species indicates that Wolbachia have been present as a potential isolating mechanism for a substantial period of evolutionary time. Finally, we argue that although Wolbachia by themselves are unlikely to bring about speciation, they can increase the rate of speciation in insects.     

TEMPERATURE-BASED EXTRINSIC REPRODUCTIVE ISOLATION IN TWO SPECIES OF DROSOPHILA

Drosophila santomea and D. yakuba are sister species that live on the African volcanic island of São Tomé, where they are ecologically isolated: D. yakuba inhabits low-altitude open and semiopen habitats while D. santomea lives in higher-elevation rain and mist forest. To determine whether this spatial isolation reflected differential preference for and tolerance of temperature, we estimated fitness components of both species at different temperatures as well as their behavioral preference for certain temperatures. At higher temperatures, especially 28°C, D. santomea was markedly inferior to D. yakuba in larval survival, egg hatchability, and longevity. Moreover, D. santomea females, unlike those of D. yakuba , become almost completely sterile after exposure to a temperature of 28°C, and conspecific males become semisterile. Drosophila santomea adults prefer temperatures 2–3°C lower than do adults of D. yakuba . Drosophila santomea , then, is poorly adapted to high temperature, partially explaining its restriction to cool, high habitats, which leads to extrinsic premating isolation and immigrant inviability. Rudimentary genetic analysis of the interspecific difference in egg hatchability and larval survival showed that these differences are due largely to cytoplasmic effects and to autosomal genes, with sex chromosomes playing little or no role.     

MOLECULAR PHYLOGENETICS AND EVOLUTION OF SEXUAL DICHROMATISM AMONG POPULATIONS OF THE YARROW'S SPINY LIZARD (SCELOPORUS JARROVII)

Understanding evolution of geographic variation in sexually dimorphic traits is critical for understanding the role that sexual selection may play in speciation. We performed a phylogenetic analysis of geographic variation in sexual dichromatism in the Yarrow's spiny lizard (Sceloporus jarrovii), a taxon that exhibits remarkable diversity in male coloration among populations (e.g., black, red, green, yellow, blue, brown). An mtDNA phylogeny based on approximately 880 bp from the 12S ribosomal RNA gene and 890 bp from the ND4 gene was reconstructed for 30 populations of S. jarrovii and eight other species of the torquatus species group using maximum-likelihood and parsimony methods. The phylogeny suggests that S. jarrovii consists of at least five evolutionary species, none of which are sister taxa. Although intraspecific diversity in male coloration is less than indicated by previous taxonomy, two species formerly referred to as S. jarrovii exhibit impressive geographic variation in sexual dichromatism. In one of these species, the phylogeny shows the independent evolution of a distinctive blue color morph in different parts of the species range. This pattern suggests that sexual selection may lead to striking phenotypic divergence among conspecific populations and striking convergence. Results also demonstrate the importance of a phylogenetic perspective in studies of evolutionary processes within nominal species and the problematic nature of “polytypic” species recognized under the biological species concept.     

THE RATE TEST OF SPECIATION: ESTIMATING THE LIKELIHOOD OF NON‐ALLOPATRIC SPECIATION FROM REPRODUCTIVE ISOLATION RATES IN DROSOPHILA

Among the most debated subjects in speciation is the question of its mode. Although allopatric (geographical) speciation is assumed the null model, the importance of parapatric and sympatric speciation is extremely difficult to assess and remains controversial. Here I develop a novel approach to distinguish these modes of speciation by studying the evolution of reproductive isolation (RI) among taxa. I focus on the Drosophila genus, for which measures of RI are known. First, I incorporate RI into age‐range correlations. Plots show that almost all cases of weak RI are between allopatric taxa whereas sympatric taxa have strong RI. This either implies that most reproductive isolation (RI) was initiated in allopatry or that RI evolves too rapidly in sympatry to be captured at incipient stages. To distinguish between these explanations, I develop a new “rate test of speciation” that estimates the likelihood of non‐allopatric speciation given the distribution of RI rates in allopatry versus sympatry. Most sympatric taxa were found to have likely initiated RI in allopatry. However, two putative candidate species pairs for non‐allopatric speciation were identified (5% of known Drosophila). In total, this study shows how using RI measures can greatly inform us about the geographical mode of speciation in nature.     

SEX AND SPECIATION: GENETIC ARCHITECTURE AND EVOLUTIONARY POTENTIAL OF SEXUAL VERSUS NONSEXUAL TRAITS IN THE SIBLING SPECIES OF THE DROSOPHILA MELANOGASTER COMPLEX

Phenotypic divergence in the male reproductive system (genitalia and gonads) between species of the Drosophila melanogaster complex and their hybrids was quantified to decipher the role of these traits in species differentiation and speciation. Internal as well as external, sexual and nonsexual traits were analyzed with respect to genetic variation and trait asymmetry between strains within species, genetic divergence between species, and dominance and asymmetry in species and hybrids. The variation between strains within species was significant among sexual traits, and only external traits were less asymmetric than internal ones, which suggests that sexual traits are not strongly constrained within species. Three main findings show that sexual traits are most divergent between species: (1) testis length and area, and the area of the posterior lobe of the genital arch (sexual traits) showed the highest proportion of variation between species; (2) linear discriminant functions with the highest components associated to sexual traits were better predictors of species membership; and (3) testis length and area revealed a departure from a linear relationship between members of the species group. Examination of interspecific hybrids showed that sexual traits had higher asymmetry in species hybrids than in the parental species and that sexual traits showed additivity or dominance whereas nonsexual traits showed overdominance (with the exception of malpighian tubules length). These results suggest that sexual traits have undergone more genetic changes and, as a result, tend to show higher divergence and stronger hybrid breakdown between species than nonsexual traits. We propose that sexual selection in the broad sense, affecting all aspects of sexuality, may be responsible for the diversified appearance of sexual traits among closely related species and that the genetic architecture underlying sexual traits may be more prone to disruption during the early stages of speciation.     

THE GENETICS OF AN ISOLATING MECHANISM BETWEEN TWO SIBLING SPECIES OF DROSOPHILA

Matings between Drosophila simulans females and males of the sibling species D. mauritiana are of abnormally short duration. These rapid matings interrupt the transfer of sperm, leading to substantial reproductive isolation in interspecific as compared to intraspecific copulations. Genetic analysis of this behavior shows that it is influenced much more by the male than the female genotype, with genes from D. simulans being dominant. In males, shortened copulation is caused by interspecific divergence at a minimum of three loci, with one gene on each of the major chromosomes. This is an underestimate of the true number of loci affecting the trait, which could be much larger. The two autosomes have the largest effect, whereas that of the X chromosome is much smaller. The genetic architecture of copulation duration and the larger effect of male than female genotype suggest that females can detect and discriminate against differences in male genitalia.     

Premating isolation is determined by larval rearing substrates in cactophilic Drosophila mojavensis. VIII. Mating success mediated by epicuticular hydrocarbons within and between isolated populations

We tested the hypothesis that intrademic sexual selection has caused sexual isolation between populations of geographically isolated populations of cactophilic Drosophila mojavensis, and was mediated by epicuticular hydrocarbons (EHCs), contact pheromones in this system. Sexual selection and sexual isolation were estimated using a Baja California and mainland population by comparing the number of mated and unmated males and females in each of four pairwise population mating trials. EHC profiles were significantly different in mated and unmated males in the interdemic (Bajafemale symbol x Mainlandmale symbol and Mainlandfemale symbol x Bajamale symbol), but not the intrademic mating trials. A small number of EHCs was identified that best discriminated among mated and unmated males, mostly alkadienes with 34 and 37 carbons. Females showed population-specific preferences for male EHC profiles. However, EHC profiles between mated and unmated males in the intrademic mating trials were not significantly different, consistent with undetectable sexual selection estimated directly from numbers of copulating pairs vs. unmated adults. Thus, sexual isolation among populations was much stronger than sexual selection within these populations of D. mojavensis.     

INTRINSIC REPRODUCTIVE ISOLATION BETWEEN TWO SISTER SPECIES OF DROSOPHILA

Drosophila santomea and D. yakuba are sister species that live on the volcanic African island of São Tomé. Previous work has revealed several barriers to gene flow, including sexual isolation, hybrid sterility, and “extrinsic” ecological isolation based on differential adaptation to and preference for temperature. Here, we describe several new “intrinsic” barriers to gene flow—barriers that do not depend on the species’ ecology. These include reduced egg number, reduced egg hatchability, and faster depletion of sperm in interspecific compared to intraspecific matings. Further, hatching interval and egg‐to‐adult development time are significantly longer in interspecific than intraspecific crosses. If a female of either species is initially mated to a heterospecific male, she lays fewer and less‐fertile eggs than if she is first mated to a conspecific male, so that heterospecific matings permanently reduce female fertility. Finally, D. santomea females mated to D. yakuba males do not live as long as virgin or conspecifically mated females. The “poisoning” effects of heterospecific ejaculates may be byproducts of antagonistic sexual selection. Although these species diverged relatively recently, they are clearly separated by many isolating barriers that act both before and after mating.     

CYTOLOGICAL CHARACTERIZATION OF PREMEIOTIC VERSUS POSTMEIOTIC DEFECTS PRODUCING HYBRID MALE STERILITY AMONG SIBLING SPECIES OF THE DROSOPHILA MELANOGASTER COMPLEX

In accordance with Haldane's rule, hybridizations between species of the Drosophila simulans clade produce fertile females but sterile males. In this study, a comprehensive characterization was undertaken on the six types of F₁ males that were the result of the crosses between D. simulans, D. sechellia, and D. mauritiana. With the use of light and electron microscopy, it was shown that while each particular hybrid genotype exhibited a specific sterility phenotype, these phenotypes fell into two distinct classes. The two hybrid genotypes that possessed D. mauritiana X-chromosomes contained spermatogenic defects that caused arrests in premeiotic spermatogenic stages. The other four F₁ hybrids possessed postmeiotic spermatogenic defects. Nonsynchronous cell divisions, underdeveloped mitochondrial derivative-axonemal associations, and microtubule abnormalities were common to all of these hybrids. Each particular postmeiotically defective hybrid genotype demonstrated characteristically distinct profiles in sperm bundle number in addition to characteristic spermiogenic arrests in the furthest developed spermatids. These results in species hybrids contrast with the absence of significant differences in spermatogenic characters between species of this clade. In addition, by utilizing an attached-X cross, we investigated the influence of maternal effects and cytoplasmic factors on the sterility of D. simulans F₁ hybrids and found none. However, we discovered a strain of D. simulans (2119) that caused a large shift in sterility from postmeiotic to premeiotic when crossed to D. sechellia. This suggests that D. simulans is polymorphic for genes involving premeiotic and postmeiotic sterility and that the two types of sterilities between species may have a simple genetic basis.     

FOUNDER-FLUSH SPECIATION IN DROSOPHILA PSEUDOOBSCURA: A LARGE-SCALE EXPERIMENT

A founder-flush-crash model of speciation has been proposed that may particularly apply to island and other colonizations. Previous laboratory experiments testing the model have given inconsistent results. We have conducted a large experiment with Drosophila pseudoobscura designed to meet the essential postulates of the model and to separately test some of the postulates. Forty-five experimental and 12 control populations have been studied during seven successive founder-flush-crash cycles, or about 50 generations. Sexual isolation tests yield significantly positive assortative mating in a few tests between pairs of experimental populations. Populations with fewer founders (N = 1 or 3) yield more significant instances of assortative mating than those with more founders (n = 5, 7, or 9), and this difference becomes statistically significant for pooled data. Only one of 15 population pairs tested three times (generations 25, 32, and 46) shows positive assortative mating in all three tests. No significant assortative mating occurs between control populations, including highly inbred ones. We conclude that although founder events may occasionally lead to the evolution of assortative mating and hence to speciation, our results do not support the claim that the founder-flush-crash model identifies conditions very likely to result in speciation events.     

SELECTION FOR SEXUAL ISOLATION BETWEEN GEOGRAPHIC FORMS OF DROSOPHILA MOJAVENSIS. I. INTERACTIONS BETWEEN THE SELECTED FORMS

Drosophila mojavensis comprises three geographic forms occurring in the United States, the Baja California peninsula, and mainland Mexico. Peninsular and mainland forms were selected for increased sexual isolation from each other, while controls were maintained with maximum outbreeding. Response to selection was highly asymmetrical in that isolation was very high between selected peninsular males and mainland females, but nonexistent between selected mainland males and peninsular females. The heightened isolation is primarily due to some change in the peninsular males.